Using ArcGIS to Analyze and Predict Invasions

Key Investigation Questions:

What are the factors limiting Tamarix's spread through the entire United States?

How are scientists using Satellite imagery to aid in predicting Tamarix's next move?

Introduction

Tamarix, or saltcedar, grows as a woody shrub or small tree. It was introduced to the United States in the 1850's as an ornamental plant capable of erosion control. It successfully adapted and quickly spread into wetlands displacing native trees such as cottonwood, willow and mesquite. Now, Tamarix has invaded nearly all the watercourses and wetlands in the Southwest US. As one of the most successful and common invasive species in the United States, it has overtaken nearly one million acres of habitat. Since it can transpire as much as 300 gallons of water per day, it is capable of causing a wetland to completely dry up. Additionally, Tamarix has a tendency to increase the salinity of the soils in which it grows and it is of little or no value to wildlife. Although Tamarix is difficult to eradicate, in locations where it has been eliminated native vegetation and wildlife have returned and flourished.

Where is the Tamarix invasion the most severe?

Tamarix can be found throughout the Southwest United States. Some states have more of a Tamarix problem than others, but because Tamarix spreads easily and is very difficult to eradicate, plant biologists are keeping a close eye on the situation.

Zoom and Pan around the map to get a better idea of the extent of the invasion and to answer the following questions.

In general, Tamarix follows rivers.Note: The US Rivers layer in AEJEE does not contain all rivers in the United States. So although you see Tamarix in parts of Nevada and California, the map is not displaying the invaded river.

Use the Selection, Select By Location menu option to select the records from the Tamarix database that are contained within the boundaries of the state of Arizona. This is a type of spatial query called "containment".

Click on the Selection menu option at the top of the map view. A pulldown context menu will open.

Click on Select by Location.

In the Select By Location dialog box, set the "I want to:" pulldown arrow to "select features from."

In "the following layer(s):" Place a checkmark in the box in front of Tamarix.

Add Satellite Data to Connect Tamarix Spread with Vegetation Index

By using the satellite data of vegetation health the scientists at NASA and USGS are able to use habitat preferences, such as moist soils and low elevations, to begin to predict where Tamarix plants were most likely to appear in the future.

Adjust the layers on the map for the investigation.

Turn off the US States, Countries and AZ Rivers layers.

Collapse the US States layer.

Turn on the vegetation index image file NDVI_6.1.09.TIFF.

This file, named NDVI_6.1.09.TIFF, shows where the vegetation has "greened up" with spring vegetation. It is a geospatially calibrated image file, or GEO TIFF, from June 6th, 2009. It was downloaded from NASA NEO.

Right click on the Tamarix layer and choose Zoom to Layer.

If necessary, adjust the layers so that the NDVI_6.1.09.TIFF, image is below the Tamarix layer.

Zoom and Pan to compare the location of the Tamarix points to the green regions of the image. Areas with healthy vegetation are prime locations for new invasions of Tamarix.

The colors on these maps show a measure of the "greenness" of Earth's landscapes. The values on these maps – ranging from -0.1 to 0.9 – have no unit. Rather, they are index values in which higher values (0.4 to 0.9) show lands covered by green, leafy vegetation and lower values (0 to 0.4) show lands where there is little or no vegetation.

As can be seen through a prism, many different wavelengths make up the spectrum of sunlight. When sunlight shines on objects, certain wavelengths are absorbed and other wavelengths are reflected. The pigment in plant leaves – chlorophyll – strongly absorbs visible light for use in photosynthesis. The cell structure of the leaves, on the other hand, strongly reflects near-infrared light. The more leaves a plant has, the more these wavelengths of light are affected. Scientists exploit this knowledge of plants' interactions with light to map the density of green vegetation across Earth's landscapes by designing satellite sensors to measure the wavelengths of red and near-infrared light that is absorbed and reflected by plants all over the world.

Subtracting plants' reflectance of red light from near-infrared light and then dividing that difference by the addition of the red and near-infrared light reflected produces a resulting value that scientists call Normalized Difference Vegetation Index (NDVI). The NDVI maps shown here were made using data collected by the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard NASA's Terra satellite.

Plants are sensitive to their environment and so they serve as a good indicator whenever there is change. Scientists routinely produce global NDVI maps to help them monitor and investigate shifts in plant growth patterns that occur in response to climate changes, environmental changes, and changes caused by humans. Farmers and resource managers also use NDVI maps to help them monitor the health of our forests and croplands. So these maps are useful both for scientific research as well as societal benefit.

Plants are sensitive to their environment and so they serve as a good indicator whenever there is change. Scientists routinely produce global NDVI maps to help them monitor and investigate shifts in plant growth patterns that occur in response to climate changes, environmental changes, and changes caused by humans. Farmers and resource managers also use NDVI maps to help them monitor the health of our forests and croplands. So these maps are useful both for scientific research as well as societal benefit.

Notice that the in Arizona the Tamarix invasion prefers the lower elevations where there is more desert. It is found in the river corridors and other low lying wet areas.

Add Elevation to the Map to Solve the Puzzle

So far we have observed that Tamarix invasion patterns follow the river corridors and also other areas that appear to have healthy green vegetation in the satellite NDVI image. So, why doesn't Tamarix invade everywhere? What is the missing element in the puzzle?

Turn on the Elevation > 2km layer. Zoom To Layer.

Use the Zoom and Pan tools to look at the map. Are there many Tamarix sightings at elevations above 2 km? What climate conditions change at this elevation?